The invention relates to a hot gas reciprocating machine, comprising at least one working space in which a working medium performs a thermodynamic cycle, said working space comprising a compression space and an expansion space of mutually different mean temperatures during operation which are connected to each other via head exchangers, including a regenerator; and more particularly, to such a machine having at least one free piston which is reciprocatable in a cylinder, one piston surface varying the volume of the working space, and the other surface forming a part of the boundary of a buffer space which also contains working medium during operation at an at least substantially constant pressure which corresponds to the mean working medium pressure in the working space. Such a machine also has a control mechanism for maintaining a given central position of the free piston by the instantaneous opening of a connection between the buffer space and a space in which a variable pressure prevails, said control mechanism comprising two elements which are constructed as an auxiliary cylinder and an auxiliary piston which is movable therein so as to vary the volume of the auxiliary cylinder space, one of these elements being connected to the free piston and the other element being rigidly arranged, the auxiliary piston having at least one first duct whose end opens through the auxiliary piston wall opposite with the auxiliary cylinder wall, where it corresponds, in a given position of the auxiliary piston, to at least one second duct in the auxiliary cylinder wall which communicates with the buffer space.
Within the scope of the present invention, hot gas reciprocating machines are to be understood to mean cold gas regrigerators, hot gas engines and heat pumps.
A hot gas reciprocating machine of the described kind has been proposed in the non-prepublished Netherlands Patent Application No. 7,514,812 to which U.S. Pat. No. 4,058,382 corresponds (FIGS. 4 and 5).
In the proposed hot gas reciprocating machine, the end of the duct in the auxiliary piston which is remote from the buffer space opens into the working space of the machine, so that a given central position of the free piston is maintained in that an open connection exists instantaneously between the buffer space and the working space. Depending on the situation, working medium then flows from the buffer space to the working space or in the reverse direction.
This construction has a drawback in that the thermodynamic cycle taking place in the working space is adversely affected. The maximum/minimum pressure ratio of the working medium participating in the cycle. is affected while at the same time a phase shift occurs, i.e. the phase difference between the pressure variation and the volume variation of the working medium in the working space changes. This leads to a reduction of the efficiency of the machine.